Basically, bar code readers operate by scanning light over a bar code which consists of a series of black and white bars. The light returned from the bar code is detected by the bar code reader and converted to an electrical signal. This signal is then processed to read the bar code. A variety of factors can create false reads, thus producing an incorrect output.
For example, most bar codes readers have had difficulties in determining when a bar code sequence begins. Often the light from the bar code reader moves from a grey background area to a highly reflective white zone before encountering the first black bar. The initial transition from the grey background to the white zone can generate a pulse in the signal which is falsely read as a black bar. As a result, the bar code reader outputs the wrong bar code.
Not only have most bar code readers had difficulties in determining the starting point of a bar code, they have also had similar difficulties in determining when to stop. Accordingly, some bar scanners accidentally scan an additional false bar which also produces an incorrect output.
Additionally, most bar code readers have failed to provide users with the ability to control externally the threshold levels for the bar code reader to detect a black bar or to control when the output of the bar code digitizer is blocked. With some applications, an external adjustment of the threshold level would be desirable because the readers could be adjusted to particular bar code labels. In other applications, it would be desirable to be able to shut off the output of a digitizer both during and after a scan.
One example of a prior bar code reader with control circuitry is U.S. Pat. No. 5,142,130 to Sato. Sato discloses a bar code reading system and bar code reading apparatus with start margin or zone detection circuitry. The start margin recognizing circuitry first checks the width of the white bar. If the width of the white bar is not normal, then the start margin detection circuitry judges that there is a black bar where the signal has a steep increase in the signal level, above a predetermined level, and that the prior increase was noise. Sato does not consider the problems discussed above, such as determining when an actual bar code read begins. Sato's start margin detection circuitry can not determine when a read begins since, it can only attempt to determine erroneous readings between black bars, by measuring the width of the separating white bar.
Another example of a prior bar code reader with control circuitry is U.S. Pat. No. 5,103,080 to Barkan which discloses a digitizer for a bar code. The circuit includes a retriggerable one-shot circuit triggered by a signal from the scanner. The one-shot circuit does not time out as long as a series of pulses from the digitizer that exceed the threshold of the one-shot circuit. There is no assurance that an actual bar is being detected in the control circuit of the Barkan patent and the circuit does not provide any means of externally setting the threshold level or allowing for any type of external control over the output of the bar code digitizer.
The present invention provides for use in a bar code reader, improved circuitry for inhibiting false and erroneous bar code reading.